In the production of olefins, ethylene in particular, a typical hydrocarbon stream like ethane, propane, butane, naphtha and gas oil, is pyrolyzed at high temperatures in a thermal furnace. The product is a mixture of olefins which are separated downstream. In the production of ethylene, typically water is co-injected with the hydrocarbon feed to act as a heat transfer medium and as a promoter of coke gasification. Typically, a minor but technologically important byproduct of hydrocarbon steam cracking is coke. Steam from the water co-injected reacts with the coke to convert it partially to carbon monoxide and hydrogen. Because of the accumulative nature, coke deposits build up on the reactor walls thus increasing both the tube temperatures and the pressure drop across the tube. This requires shutting down the process for decoking. This periodic shutdown results in an estimated $2 billion in lost ethylene production per year. In addition there is a direct relationship between the amount of coking and the yield of the olefin, indicating that the coke is formed at the expense of product olefin.
It is common practice in commercial ethylene production to co-inject along with the hydrocarbons, small amounts of sulfur containing compounds such as hydrogen sulfide (H2S), dimethyl sulfide (DMS) or dimethyl disulfide (DMDS) to minimize coke formation. It has been proposed that the sulfur passivates the active metal surface known to be a catalyst for coke formation. In addition, the sulfur compounds are known to reduce the formation of carbon monoxide (CO), formed by the reaction of hydrocarbons or coke with steam, again by passivating the catalytic action of the metal surface and by catalyzing the water gas shift reaction which converts the CO to carbon dioxide (CO2). Minimizing the amount of CO formed is essential for the proper functioning of downstream reduction operations.
U.S. Pat. No. 4,404,087 discloses that pretreating cracking tubes with compositions containing tin (Sn) compounds, antimony (Sb) and germanium (Ge) reduces the rate of coke formation, during the thermal cracking of hydrocarbons.
Combinations of Sn, Sb and Si are disclosed to do the same in U.S. Pat. No. 4,692,234.
Mixtures of chromium and antimony compounds, chromium and tin compositions, and antimony and chromium compositions have also been claimed to reduce coke formation, as measured by a time weighted CO selectivity index (U.S. Pat. No. 4,507,196).
Several phosphorous and sulfur compound combinations are disclosed (U.S. Pat. No. 5,954,943) with Sn and Sb compounds (U.S. Pats. Nos. 4,551,227; 5,565,087 and 5,616,236), for decreasing the coke formed in hydrocarbon pyrolysis furnaces.
In general, U.S. Pats. Nos. 4,507,196; 4,511,405; 4,552,643; 4,613,372; 4,666,583; 4,686,201; 4,687,567; 4,804,487; and 5,015,358, teach that the metals Sn, Ti, Sb, Ga, Ge, Si, In, Al, Cu, P, and Cr, their inorganic and organic derivatives, individually or as mixtures will function as antifoulants for the reduction of coke during hydrocarbon pyrolysis.
Phosphoric acid and phosphorous acid mono and di-esters or their amine salts, when mixed with the feed to be cracked, for example, ethane, showed a significant increase in run lengths compared to an operation performed without the additives (U.S. Pat. No. 4,105,540).
Pretreating furnace tubes at high temperature with aromatic compounds such as substituted benzenes, naphthalenes and phenanthrenes, prior to introduction of the cracking feed has been shown to reduce catalytic coke formation (U.S. Pat. No. 5,733,438). Cracking a heavy hydrocarbon, preferably a higher olefin stream prior to bringing on the lower hydrocarbons, has been shown to reduce coking (U.S. Pat. No. 4,599,480). In both cases, a thin layer of catalytically inactive coke formed on the tube surface is claimed to inhibit the propagation of coke formation.
Several patents disclose the use of various Si compounds to lay down a ceramic layer on metal tubes and thus reduce the coke formed in pyrolysis. Compounds such as siloxanes, silanes and silazanes have been used to deposit a silica layer on the metal alloy tubes (U.S. Pats. Nos. 5,424,095; 5,413,813; and 5,208,069). Silicates have been independently claimed to do the same in patent GB 1552284. In almost all of the examples the coke minimization is of catalytic coke, formed mainly during the early stages of pyrolysis. A patent (U.S. Pat. No. 5,922,192), teaches the use of a silicon compound and a sulfur compound as a mixture that contains Si/S ratio of 1/1 to 5/1 to mitigate coke formation.
Another approach to reduce coking is to passivate the active metal surface of pyrolysis tubes by forming a surface alloy, comprising metals/oxides of metals that are known to not catalyze coke formation. High Temperature Alloys (HTA) are a group of austenitic stainless steels used in industrial processes operating at elevated temperatures above 650° C. These typically contain 118-38% Cr, 18-48% Ni, with the balance being Fe and alloying additives. Iron and nickel are known catalysts for the formation of filamentous carbon during ethylene production and hydrocarbon pyrolysis in general. An oxide layer of chromium or aluminum on the other hand are known to be inhibitors of catalytic coke formation and thus are used to protect these alloys. Protection using these oxides have to be carefully engineered so that physical characteristics and properties of the HTA, such as creep resistance, are not compromised and the oxide layer is stable to harsh conditions typically encountered in hydrocarbon pyrolysis. CoatAlloy™ is a surface coating technology for the inside of HTA tubes for use in an ethylene furnace. Cr—Ti—Si and Al—Ti—Si formulated products are coated on a base alloy surface and heat treated to form either a diffusion protective layer only or a diffusion layer and a enrichment pool layer next to it. In both cases, oxidizing gases are passed to activate the layers by formation of alumina and/or chromia along with titania and silica. The treated tubes have been claimed to significantly reduce catalytic coke formation, minimize carburization of the base alloy tubes, exhibit improved erosion resistance and thermal shock resistance (U.S. Pat. No. 6,093,260). The ethane gas stream used to test the effectiveness of the coating contained 25-30 PPM of sulfur. A combination of sulfur containing compounds such as an alkyl mercaptan- or alkyl disulfide and a nitrogen containing compound such as hydroxylamine, hydrazine or amine oxide are disclosed as useful in pretreating or minimizing coke formation in thermal furnaces (U.S. Pat. No. 6,673,232).
Reduction of coking rates on both quartz and Incoloy surfaces by the use of low concentrations of hexachloroplatinic acid (H2PtCl6) in the steam used for ethane cracking, have been reported (Industrial & Engineering Chemistry Research, Vol: 37, 3, 901, 1998). Coke formation rates were reduced although the apparent activation energies increased. The reduced effectiveness of the additive at higher temperatures suggests that the primary impact of the additive was on the surface coke formation process.
The typical previous approaches have involved either metal passivation techniques with various additives like sulfur, silicon, phosphorous, etc., or the use of special alloys which reduce coking. These are surface treatments. The use of phosphorus containing compounds has become problematic due to adverse affects on downstream operations. Similarly, the use of amines and derivatives thereof has become problematic due to the formation of NOx and its impact on downstream operations.
The objective of the present invention was to develop improved technology for reducing the formation of coke in commercial thermal cracking furnaces. Reduced coke levels will translate into higher ethylene yields, longer radiant furnace tube life and reduced downtime for decoking of the unit which allows increased total production.